During drilling operations, there are numerous forces that act on a drill bit that can influence the drilling direction. For example, a drilling steering tool, such as a rotary steerable tool, may be impacted by lateral forces that tend to “push” steering, via the drill bit, in a particular direction.
Lateral forces include, for example, forces exerted on the drill bit by the formation through which drilling is taking place. During straight drilling, lateral forces can result from such causes as anomalies in the formations being drilled, formation anisotropy, imbalances in the drill string, the arrangement of components within the drill string, and as a reaction to rotation of the drill bit (also referred to colloquially as “bit walk”). During directional drilling, lateral forces may additionally result from reaction forces exerted by the formation in resistance to the steering tool's lateral push to change the direction of drilling. These lateral forces exerted by the formation against the drill bit, and in turn the steering tool, are referred to generally as “formation tendency.”
Directional steering of the drill can be carried out in several ways. For example, in a “push the bit” system, the drill bit is pushed laterally in the desired direction. In a “point-the-bit” system, the drill bit is pointed in the desired direction by changing the orientation of the drill bit axis relative the borehole. In both systems, for steering purposes, it is typically assumed that drilling proceeds in the direction the drill bit is pushed or pointed, and that the borehole exerts a reaction force on the steering tool in a direction directly opposite to the direction in which the drill bit is being pushed or pointed.
However, the indeterminate lateral forces noted above push and pull on the drill bit, via the toolface, altering the steering direction and causing drilling to veer off course. Therefore, an operator may intend to drill in one direction toward a target, yet due to these lateral forces, drilling proceeds off course.